Catalytic cracking is a petroleum refining process which is applied commercially on a very large scale. A majority of the refinery gasoline blending pool in the United States is produced by this process. In the catalytic cracking process heavy hydrocarbon fractions are converted into lighter products by reactions taking place at elevated temperature in the presence of a catalyst, with the majority of the conversion or cracking occurring in the vapor phase. The feedstock is thereby converted into gasoline, distillate and other liquid cracking products as well as lighter gaseous cracking products.
During catalytic cracking, heavy material, known as coke, is deposited onto the catalyst. This reduces its catalytic activity and regeneration is desired. After removal of hydrocarbons from the spent cracking catalyst, regeneration is accomplished by burning off the coke which restores the catalyst activity. The three characteristic steps of the catalytic cracking can be therefore be distinguished: a cracking step in which the hydrocarbons are converted into lighter products, a stripping step to remove hydrocarbons adsorbed on the catalyst and a regeneration step to burn off coke from the catalyst. The regenerated catalyst is then reused in the cracking step. Catalytic cracking feedstocks normally contain sulfur in the form of organic sulfur compounds such as mercaptans, sulfides and thiophenes. The products of the cracking process correspondingly tend to contain sulfur impurities even though about half of the sulfur is converted to hydrogen sulfide during the cracking process.
For modern refineries, the Fluid Catalytic Cracking Unit (FCCU) produces 40 to 60+% of the gasoline in the gasoline pool. In addition, the FCCU produces a blendstock component for diesel manufacture. Air quality regulations for these transportation fuels will require a further reduction in sulfur content as mandated by the Clean Air Act. For the FCCU process, there are two routes a refiner can utilize to further reduce the sulfur content of these transportation fuels. The first route is via a hydrotreatment process on the feedstock to the FCCU. This hydrotreatment process can by operational severity and design, remove a substantial amount of the feed sulfur to produce a gasoline sulfur content of 100 ppmw or less. The second route a refiner can take involves the use of a specialized catalyst or additive in the FCCU circulating catalyst inventory that can catalytically remove sulfur from the FCCU product distributions. Refiners may elect to use this route for both non-hydrotreated and/or hydrotreated FCCU feedstock derived from various crude sources. In addition, if a refiner utilizes the first route for desired gasoline sulfur content, when the hydrotreater is taken out of service for an outage, this specialized catalyst or additive can be utilized to minimize the increase of gasoline sulfur during the outage period.
A need exists to continue to remove SO2 gas. A need also remains in the refining industry for improved compositions and processes which minimizes the content of gas phase reduced nitrogen species and NOx emitted from a partial or complete combustion FCCU riser during an FCC process, which compositions are effective and simple to use.
Other objects and advantages of the present invention will become apparent to those skilled in the art upon a review of the following detailed description of the preferred embodiments and the accompanying drawings.